1. Field of the Invention
The present invention relates to a field isolating device for a Wien filter, and more particularly to a design of Wien Filter or called as ExB for all the applications, such as deflecting charged particle beam or separating charged particles in dependence upon mass, or energy, or moving direction thereof in an electron microscope.
2. Description of the Prior Art
Wien filter is known for its capability to separate charged particles in dependence upon mass, or energy, or moving direction thereof. It is based on the principle that the magnetic force of a magnetic field acting on a charged particle depends on the velocity vector thereof, but the electric force of an electric field acting on a charged particle does not.
Concretely speaking, a Wien filter comprises an electric deflector and a magnetic deflector, both of which share a common straight optical axis. The electric dipole field E1 and the magnetic dipole field B1 respectively generated by the electric and magnetic deflectors are perpendicular to each other, and superimposed along and perpendicular to the common straight optical axis. When a particle enters the Wien filter along the optical axis, if the vector directions of the electric and magnetic fields E1 and B1 and the particle velocity ν obey the right-hand rule, the electric force and magnetic force acting on the particle will be opposite to each other and perpendicular to the moving direction of the particle, hence generating a total force as shown in Equation (1.1).F=q·(E1−ν·B1)  (1.1)
Furthermore, if the magnitudes of the foregoing three vectors meet a specific condition which is called as Wien Condition and shown in Equation (1.2), the net force exerted on the particle will be zero, thereby not changing the moving of the particle. Any other particle, different from the particle in velocity vector, will receive a non-zero net force, and thereby being deflected away from its original moving direction.E1−ν·B1=0  (1.2)
Considering the relationship of velocity ν, mass m and kinetic energy V of a particle, which is shown in Equation (1.3), obviously, Wien Condition can be satisfied for particles having a given ratio between mass and energy and a given moving direction. Therefore many applications of Wien filter have been proposed for separating particles identical in charge and energy but different in mass (such as a mass filter), or particles identical in charge and mass but different in kinetic energy (such as an energy filter), or two charged particle beams both having particles identical in mass and charge but traveling in mutually opposite directions, such as primary beam and secondary beam in SEM.
                    v        =                                            2              ·              V                        m                                              (        1.3        )            
In most of those applications, Wien filter is employed in an imaging system, wherein the straight optical axes of the Wien filter and the imaging system coincide. If the Wien Condition is not satisfied wherever the particles in the imaging beam will travel through, the additional aberrations will be added to the imaging beam due to the undesired particle deflection. Therefore, constraining or even eliminating the adverse impact of Wien filter on imaging quality is a prerequisite for employing a Wien filter in such a case, and meeting Wien Condition to the maximum extent possible is the essential requirement for practically constructing a Wien Filter. Practically meeting Wien Condition can be considered in terms of the on-axis area (on the optical axis) and the off-axis area (off the optical axis).
At first, in the on-axis area, the velocities of on-axis particles are constant because there is no axial acceleration or deceleration field within Wien filter. Therefore Equation (1.2) requires the on-axis electric and magnetic fields E1 and B1 have a same distribution shape. If it is not true, the net forces exerted on the on-axis charged particles will not be zero, thereby gradually deflecting the particles away from the optical axis and generating off-axis aberrations. The better the two fields match each other in field distribution shape, the smaller the net forces will be, thereby appearing the less moving-direction deviation from the optical axis and the off-axis aberrations.
Secondly, in the off-axis area, due to a potential change in the electric field direction, an off-axis particle in the imaging beam will have a velocity not only different from the given velocity of the on-axis particles but also dependent on its off-axis shift. Therefore, if the magnitudes of the electric and magnetic dipole fields have a uniform distribution in the electric field direction, the Wien Condition can not be satisfied over the entire off-axis area, thereby leading to a focusing effect in this direction and hence adding astigmatism to the imaging particle beam.
A number of methods for practically constructing a Wien filter have been proposed by taking account of the foregoing issues. An effective and advantageous way is to construct either or both of the electric and magnetic deflectors with a multi-pole structure. The multi-pole structure can generate a dipole field and a quadrupole field simultaneously, which enables the astigmatism being compensated where it occurs. Furthermore, the two fields can be electrically adjusted separately. Therefore such a Wien filter is easily adjusted to match the changeable operation conditions of the imaging system which the Wien filter is applied to. To obtain a good fields-match of the electric and magnetic dipole fields in field distribution shape, the electric and magnetic deflectors are even constructed to have a common multi-pole structure, wherein all the magnetic pole-pieces act as electrodes as well, such as the 8-pole type Wien filter proposed by Tian-Tong Tang (Optik, 74, No. 2, 1986, P51-56). This design fundamentally ensures the fields-match is good inside the electric and magnetic deflectors.
However, for a Wien filter having multi-pole structure, an issue related to higher order harmonics appears. When a multi-pole device is excited to generate a dipole field, it actually generates a field which not only comprises a dipole field or called as 1st order harmonic but also many higher order harmonics which are undesired due to incurring aberrations. To minimize the first higher order harmonic, i.e., 3rd order harmonic, a number of measures on arranging and exciting the multiple magnetic pole-pieces or electrodes have been proposed. For the Wien filters whose electric and magnetic deflectors have a common multi-pole structure, Lopez and Tsuno proposed a 12-pole structure with a 12-fold rotational symmetry in U.S. Pat. No. 6,844,548. For the Wien filters in which the electric and magnetic deflectors are separate in structure, Chen et al. file a U.S. patent application Ser. No. 13/292,455, filed Nov. 9, 2011 to provide a solution which is shown in FIGS. 1(a)-1(d). In this Wien filter 100 which has an optical axis on Z-axis, the cylindrical 12-electrode electric device 30 and the cylindrical 4-coil magnetic device 40 act as an electric deflector and a magnetic deflector respectively. The special 8-fold symmetry in geometry as shown in FIG. 1(a) and the special 2-fold symmetry in potential distribution of the device 30 as shown in FIG. 1(c) ensure it generates an electric dipole field E1 in X direction and limits the 3rd order harmonic negligibly small. The special 4-fold symmetry in geometry as shown in FIG. 1(a) and the combination of a 2-fold symmetry and a 2-fold anti-symmetry in magnetomotive force distribution of the device 40 as shown in FIG. 1(c) ensure it generates a magnetic dipole field B1 in Y direction and limits the 3rd order harmonic negligibly small.
For the applications of Wien filter in an imaging system having a high imaging resolution, such as beam separator and Monochromator in a high resolution scanning electron microscope (SEM), the foregoing solutions are not enough. Meeting Wien Condition more strictly becomes more necessary. Among all the necessary improvements, the deviation of the fields-match from a perfect match in the fringe areas on both sides of a Wien filter deserves the first attention. The deviation is simply called as the fields-mismatch and expressed by the difference between the normalized on-axis magnetic and electric fields B1 and E1 hereafter.
In U.S. patent application Ser. No. 13/292,455, filed Nov. 9, 2011, Chen et al. further propose a measure to reduce the fields-mismatch in the fringe areas of the Wien filter, which is shown in FIG. 1(b). The two conical end portions of the electric device 30 and two field-terminating plates 61 and 62 ensure the fields-mismatch small within the Wien filter 100. The two field-terminating plates 61 and 62 are made of a material of both electric and magnetic conductor. The higher their permeability u21 and u22 are, the smaller the fields-mismatch in the two fringe areas 21 and 22 will be. FIG. 1(d) shows the fields-mismatch when u21 and u22 both are same and equal to 1000 and 10000 respectively. Increasing permeability u21 and u22 effectively reduces the fields-mismatch in the fringe areas 21 and 22, i.e. the areas at entrance and exit ends of the Wien filter. For the fringe areas 23 and 24 outside but close to the Wien filter, the fields-mismatch is still large and almost not reduced. The concrete analysis shows the fields-mismatch in the fringe areas 23 and 24 comes from the non-zero magnetic dipole field in these areas. For the sake of the clarity, 21 and 22 are called as the near fringe areas, and 23 and 24 as the far fringe areas. However, the fields-mismatch in the far fringe areas 23 and 24 will gradually deflect the particles away from the optical axis. For the imaging beam, the deflection accumulation happened before and after the Wien filter will make the beam enter the Wien filter and the other following imaging elements not along the optical axis, and it may eventually result in a non-negligible deviation of the landing position and large off-axis aberrations of the imaging beam on the destination plane.
Accordingly, a field-isolating device for a Wien filter, which can reduce or even eliminate the magnetic field leaking out of a Wien filter onto the far fringe areas thereof, is demanded by many applications of Wien filter, particularly for a charged particle apparatus using Wien filter in its imaging system such as a SEM. In addition, as mentioned in the cross-reference, a new design of Wien filter, which has a compact and efficient structure that can have much flexibility to meet the Wien Condition as much as possible within the Wien filter, is also necessary for the foregoing applications.